There have been temperature indicators that indicate particular temperature ranges without a use of a temperature measurement mechanical device or a mercury thermometer or the like for the purpose of temperature management of machines, installations or constant temperature preservation products. As such temperature indicators, there are known temperature indicators which indicate color tones corresponding to the temperatures by means of a temperature indicating member utilizing a temperature indicating pigment which undergoes reversible color changes. These temperature indicators are used for the purposes of temperature management of a mechanical installation and burn prevention and the like, for the reason that it is possible to monitor the real-time temperature with ease through observation of the reversible variation in color tone. In the former time, a mercury-containing halogen complex compound was commonly adopted as the temperature indicating material used in such temperature indicating members. However, in recent years this use has been progressively abandoned from the viewpoint of protection of human health and environment against mercury.
As a reversible temperature indicator not including mercury, there have been known, for example, ones making use of electron-donating organic compound or such organic pigment which reversibly exhibit colors in response to temperatures and ones making use of organic paints contained in microcapsules; such examples are described in Patent Documents 1 through 4 listed below. However, generally speaking, organic compounds have a tendency to decompose with time, so that the indicators making use of temperature indicating materials made of the organic compounds, or ones making use of microcapsules containing the organic compounds, and ones carrying organic compounds coated with polyester film are apt to be unsatisfactory in terms of durability, especially light stability.
Also, as a reversible temperature indicator not including mercury, there have been known, for example, a temperature indicating member containing cobalt compounds which reversibly exhibit colors in response to temperatures, as described in Patent Documents 5 and 6. Compared with the temperature indicating members containing mercury-containing halogen complex compound, the temperature indicating members containing such cobalt compounds are designed with more consideration for the protection of human body and environment, and as such they are safe and useful up to a certain level, but from the fact that cobalt compounds are themselves environmentally hazardous substances. Therefore, there has been a concern that their use would not sustain ecological safety and environmental protection. For example, in EU's Registration, Evaluation, Authorisation and Restriction of Chemicals (REACH), cobalt(II) chloride, which is a cobalt compound, is included in “Candidate List of Substance of Very High Concern for authorization” published by European Chemical Agency, which enumerates items to which industries are required to impart special management (ref. Nikkei Ecology, September Edition, 2008). Furthermore, a cobalt (II) triazole complex compound, in which ligand is tirazole and the central metal to which triazole is coordinated is cobalt, also performs thermochromism to reversibly change its hue in response to temperatures, as described in below-listed non-Patent Document 1, but since it also contains cobalt metal there is a possibility that it receives some sort of regulation in future.
Now, in publications such as below-listed non-Patent Documents 2 and 3, it is described that an iron triazole complex compound, which is an iron-containing complex compound, undergoes reversible color change at particular temperatures. However, powdery iron triazole complex compound is poor in durability and does not make a practical temperature indicating pigment for temperature indicating member. Also, as a functional film device utilizing an iron triazole complex compound there is described in below-listed Patent Document 7, for example, an electrooptically functional film device, which contains an iron triazole complex compound and is made into a film form. This functional film is synthesized by emulsion polymerization method so that it is necessary to control the particle diameter responsive to the applications and thus requires extra time and cost. Furthermore, with this functional film device, its portion that contains iron complex is of a discontinued semi-spherical shape, so that it incurs limitation to the design-ability of the temperature indicator material as well as cost increase. Also, as an example of an application of using an iron triazole complex compound, there is disclosed in below-listed Patent Document 8, for example, an iron triazole complex compound having a ligand of a particular highly integrated structure, which is used as a storage element and the like rather than as a temperature indicating member. And in Patent Document 9, there is disclosed an optical element having a layer containing a liquid crystal gel, which is formed by a gelled composition including a triazole metal complex and a liquid crystal compound.